Article on The Internet Mini Encyclopædia
Spanners and Sockets, Equivalent Sizes
The following table covers common (and not-so-common) nut and bolt sizes, in AF, Metric, BSW and BSF. Below the table is an explanation of why the different standards exist, and how they are often compatible. All material provided by Pete Kay (www.eraturbo.com)
|Across flats||Designated socket or spanner|
|0.3438||8.74||11/32” AF and 1/8” BSW|
|0.4450||11.30||3/16” BSW and 1/4” BSF|
|0.5250||13.335||1/4” BSW and 5/16” BSF|
|0.6000||15.24||5/16” BSW and 3/8” BSF|
|0.7100||18.034||3/8” BSW and 7/16” BSF|
|0.8200||20.828||7/16” BSW and 1/2” BSF|
|0.9200||23.368||1/2” BSW and 9/16” BSF|
|1.0100||25.654||9/16” BSW and 5/8” BSF|
|1.1000||27.94||5/8” BSW and 11/16” BSF|
|1.2000||30.48||11/16” BSW and 3/4” BSF|
|1.3000||33.02||3/4” BSW and 7/8” BSF|
|1.3900||35.306||13/16” BSW and 15/16” BSF|
|1.4800||37.952||7/8” BSW and 1” BSF|
|1.670||42.418||1” BSW and 1.1/8” BSF|
|1.8600||47.244||1.1/8” BSW and 1.1/4” BSF|
So why 4 measuring standards?
When the Mini was designed in the in the Nineteen Fifties, (when I was in my late teens) Metric threads, and spanners marked with their Across Flat (AF) size in millimetres were very rare, except in continental Europe.
Nearly all British cars (except Ford) used either British Standard Whitworth (BSW) or British Standard Fine (BSF) threads for any screw application. The former was a course thread, used for larger screws, which varied in pitch according to its diameter, and the finer threaded BSF was used for short screws, which was more applicable to motor vehicles. Both these screws have an angle of 55deg between threads, with the thread width, and hence pitch, varying in relation to diameter.
Because Ford cars were designed to American standards, they used American screw threads, which were termed National Coarse (NC) and National Fine (NF). They were similar to the English versions, but had a 66deg thread angle. Due to problems in WW2 with thread sizes, the sizes of these American threads were 'unified' between Britain, Canada and America and, from about 1960 they became UNC and UNF.
Now to the point of the story. The British retained spanners marked with BSW and BSF sizes, which related to the thread diameter, not the across flats (AF) size used commonly but incorrectly by the Americans for their UNF and UNC screws. In fact, each spanner was marked with both Whitworth and BSF sizes, which are directly related but not the same.
For any given imperial size, up to 7/8 inch BSF, the Whitworth equivalent is always 1/16th of an inch smaller than the equivalent BSF. e.g. a 1/2 inch BSF screw has the same across flats size as a 7/16inch Whitworth screw (or nut). Hence the dual marking of the spanner. For larger sizes, the difference is 1/8 inch.
Unfortunately, this does not indicate the across flats (AF) size of the screw or nut. The Americans, however did standardise the Unified screws to have a specific relationship between the screw diameter and head size across flats. This relationship is the same as that used by the English for the BSF sizes. For example, a bolt with a thread diameter of 1 inch, in either Unified or BSF has a head measurement of 1.5inch AF. Similarly a 3/4 inch diameter thread has a head which is 1.1/8 inch across flats.
Be Warned. There are not many precise equivalents from BSW/BSF to imperial AF or Metric sizes. Most nearly fit!
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